Semiconductor integrated circuit device

ABSTRACT

To a plurality of terminals are connected, one to one, a plurality of current mirror circuits each having a first load portion through which a first current flows when an overvoltage is applied and a second load portion through which a second current proportional to the first current flows. The first load portion of the current mirror circuit connected to one of the plurality of terminals is shared as the first load portions of the current mirror circuits connected to the others of the plurality of terminals. The shared first load portion includes an overvoltage protection circuit. The second load portions of the current mirror circuits connected to the plurality of terminals each include an overvoltage detection circuit for detecting the second current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a protection circuit for protecting asemiconductor integrated circuit device and its internal circuits whenan overvoltage or a surge of any kind is applied to a terminal of thesemiconductor integrated circuit device. The present invention relatesalso to a semiconductor integrated circuit device provided with such anovervoltage detection circuit.

2. Description of the Prior Art

As electronic appliances employing semiconductor integrated circuitdevices become increasingly varied, and their uses become increasinglyvaried, more and more cases are reported in which an overvoltage in theform of noise, a surge, static electricity, or in any other form flowsdirectly or indirectly, for example through a human body, into anelectric circuit, destroying an electronic appliance or inflictingserious damage thereon. Even at manufacturing sites of electronicappliances, the measures taken against such hazards are not sufficientto satisfactorily suppress rejection rates.

For this reason, in an electronic circuit or semiconductor integratedcircuit device, a terminal thereof that is likely to be exposed to anovervoltage is provided with a protection circuit to prevent anovervoltage from being applied to a functional circuit provided inside.

FIGS. 6 and 7 show conventional examples of such overvoltage protectioncircuits. FIG. 6 shows conventional overvoltage protection circuits. InFIG. 6, reference numeral 10 represents a semiconductor integratedcircuit device having terminals “a” to “m”, to each of which isconnected a protection circuit composed of n Zener diodes ZD1 to ZD(n)connected together in series, with the anode of the Zener diode at theother end grounded. Though not illustrated, to each terminal isconnected, in addition to one end of such a protection circuit, one endof an internal functional circuit.

In this circuit configuration, an internal circuit of the semiconductorintegrated circuit device 10 is protected from a voltage higher than apredetermined voltage by setting a protection voltage that is determinedby the characteristics of Zener diodes, namely, their Zener voltage andhow many of them are connected together. Specifically, to protect aninternal circuit of the semiconductor integrated circuit device 10 froman overvoltage higher than 100 V, for example, 10 Zener diodes that eachhave a Zener voltage of 10 V and that are connected together in seriesare connected to each of the terminals “a” to “m,” so that, even when anovervoltage higher than 100 V is applied to any of the terminals “a” to“m,” a reverse current flows through the Zener diodes ZD1 to ZD(n) toground, and thus no overvoltage higher than 100 V flows into theunillustrated internal circuit. On the other hand, the normal signalvoltages that are fed to the internal circuit via the terminals “a” to“m” are all lower than the protection voltage so set. Thus, for example,when a 5V signal voltage is fed to the terminal “a,” no reverse currentflows through the Zener diodes, and therefore the signal voltage isproperly fed to the internal circuit.

FIG. 7 shows conventional overvoltage protection circuits incorporatingovervoltage detection circuits. Here, such circuit elements as have thesame functions as in FIG. 6 are identified with the same referencesymbols. Reference numeral 11 represents a semiconductor integratedcircuit device having terminals “a” to “m.” To the terminal “a” isconnected a protection circuit, similar to those shown in FIG. 6,composed of n Zener diodes ZD1 to ZD(n) connected together in series,with the anode of the Zener diode at the other end grounded through aresistor R1, of which one and the other ends are connected to the baseand emitter, respectively, of an NPN-type transistor Qa. Though notillustrated, the collector of the transistor Qa is connected, forexample, to an internal circuit of the semiconductor integrated circuitdevice. The circuits for the other terminals are configured likewise.

In this configuration, how the internal circuit is protected from anovervoltage by the Zener diodes is the same as in FIG. 6, and thereforeno explanation of it will be repeated. Now, the overvoltage detectioncircuit composed of the resistor R1 and the transistor Qa will bedescribed. When an overvoltage causes an overcurrent I1 to flow throughthe Zener diodes ZD1 to ZD(n) and through the resistor R1 to ground, avoltage drop of I1×R1 occurs across the resistor R1, turning thetransistor Qa on. Accordingly, by connecting the collector of thistransistor Qa to the unillustrated internal circuit of the semiconductorintegrated circuit device, it is possible to extract an overvoltagedetection signal SA to disable the internal circuit momentarily or untilthe entire circuit is reset.

As described above, according to the conventional technique, many Zenerdiodes need to be connected to each terminal that is likely to beexposed to an overvoltage. This not only makes a semiconductorintegrated circuit device unduly large, but also increases the number offabrication steps and thus costs. Moreover, variations in thecharacteristics of the circuit elements used cause variations in theprotection voltage from one terminal to another.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductorintegrated circuit device provided with an overvoltage detection circuitthat operates with less circuit elements and thus requires a smallerchip size, thereby permitting cost reduction, and that operates withsmaller variations in the protection voltage from one terminal toanother.

To achieve the above object, according to one aspect of the presentinvention, in a semiconductor integrated circuit device including anovervoltage protection circuit and an overvoltage detection circuit fora plurality of terminals, to a plurality of terminals are connected, oneto one, a plurality of current mirror circuits each having a first loadportion through which a first current flows when an overvoltage isapplied and a second load portion through which a second currentproportional to the first current flows. The first load portion of thecurrent mirror circuit connected to one of the plurality of terminals isshared as the first load portions of the current mirror circuitsconnected to the others of the plurality of terminals. The shared firstload portion includes an overvoltage protection circuit. The second loadportions of the current mirror circuits connected to the plurality ofterminals each include an overvoltage detection circuit for detectingthe second current.

According to another aspect of the present invention, a method offabricating a semiconductor integrated circuit device including aplurality of circuits, each composed of a protection diode and a currentmirror circuit consisting of first and second transistors having basesthereof connected together, having emitters thereof connected together,and having the emitters thereof connected to an anode of the protectiondiode, and a plurality of terminals connected one to one to the circuitsincludes the steps of forming the anode of the protection diode on thesemiconductor integrated circuit device, forming the cathode of theprotection diode so as to surround the anode thereof, forming thecollector of the first transistor so as to surround about a half of theanode within a region surrounded by the cathode, forming the collectorof the second transistor so as to surround about another half of theanode, and forming the bases of the first and second transistors as acommon base so as to be substantially parallel to an edge of thecollectors of the first and second transistors.

According to the present invention described above, even if a pluralityof terminals are likely to be exposed to an overvoltage, by sharing aprotection circuit against an overvoltage, it is possible to protect theterminals individually from an overvoltage and output an overvoltagedetection signal. In addition, it is possible to reduce the number ofcircuit elements needed, miniaturize the semiconductor integratedcircuit device as a whole, reduce costs, and reduce variations in thedetection voltage resulting from variations in the characteristics ofthe circuit elements used.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of the present invention will becomeclear from the following description, taken in conjunction with thepreferred embodiments with reference to the accompanying drawings inwhich:

FIG. 1 shows an overvoltage protection circuit incorporating anovervoltage detection circuit, as a first embodiment of the invention;

FIG. 2 shows an overvoltage protection circuit incorporating anovervoltage-blowing fuse, as one example of a second embodiment of theinvention;

FIG. 3 is an overvoltage protection circuit incorporating a memory cellfor storing a history of overvoltages, as another example of the secondembodiment of the invention;

FIG. 4A is a diagram showing an equivalent circuit of the overvoltageprotection diode;

FIG. 4B is a sectional view of the semiconductor integrated circuitdevice;

FIG. 4C is a top view of the semiconductor integrated circuit device;

FIG. 5A is an equivalent circuit of the overvoltage protection diode andthe current mirror circuit used in a third embodiment of the invention;

FIG. 5B is a sectional view of the semiconductor integrated circuitdevice of the third embodiment;

FIG. 5C is a top view of the semiconductor integrated circuit device ofthe third embodiment;

FIG. 5D shows practical example 1 of the semiconductor integratedcircuit device of the third embodiment;

FIG. 5E shows practical example 2 of the semiconductor integratedcircuit device of the third embodiment;

FIG. 6 shows a conventional overvoltage protection circuit; and

FIG. 7 shows a conventional overvoltage protection circuit incorporatingan overvoltage detection circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings. FIG. 1 shows, as a first embodiment of theinvention, an overvoltage protection circuit incorporating anovervoltage detection circuit. In this figure, such circuit elements asare found in or have the same functions as in FIGS. 6 and 7 areidentified with the same reference symbols, and their basic explanationswill be omitted.

Reference numeral 20 represents a semiconductor integrated circuitdevice having terminals “a” to “m” that are likely to be exposed to anovervoltage. To the terminal “a” are connected the emitters of PNP-typetransistors Qa1 and Qa2 having their emitters connected together andhaving their bases connected together, with their bases connectedtogether to the collector of the transistor Qa1. Thus, the transistorsQa1 and Qa2 form a current mirror circuit having a first and a secondload portions 1 and 2 a on the downstream sides of the collectors of thetransistors Qa1 and Qa2, respectively. The terminal “a” is connectedalso to an internal circuit by way of an unillustrated conductor, and toa supply voltage or ground through a protection diode or a parasiticdiode.

In the first load portion 1, there are connected, on the downstream sideof the collector of one transistor Qa1, three Zener diodes ZD1 to ZD3that are connected together in series and, further on the downstreamside thereof, NPN-type transistors Tr4 and Tr5 that are connectedtogether in series and that each have their collector and base connectedtogether, with the emitter of the transistor Tr5 grounded. On the otherhand, in the second load portion 2 a, there is connected, on thedownstream side of the transistor Qa2, a resistor Ra through which thecollector of the transistor Qa2 is grounded. One end of the resistor Rais connected to the base of an NPN-type transistor Qa, and the other endof the resistor Ra is connected to the emitter of the transistor Qa.Thus, when application of an overvoltage to the terminal “a” isdetected, the transistor Qa outputs an overvoltage detection signal Sathrough its collector.

The circuits for the terminals “b” to “m” are configured as follows.Here, the circuit for the terminal “i” is taken up as therepresentative. As with the terminal “a,” to the terminal “i” areconnected the emitters, connected together, of PNP-type transistors Qi1and Qi2 forming a current mirror circuit, and the bases thereof,connected together, are connected to the first load portion, which isshared as a common circuit portion among the circuits for the otherterminals “a” to “h” and “j” to “m.” On the other hand, as with theterminal “a,” in the second load portion 2 i for the transistor Qi2, oneend of a resistor Ri is connected to the collector of the transistorQi2, and the other end of the resistor Ri is grounded, with one and theother ends of the resistor Ri also connected to the base and emitter,respectively, of an NPN-type transistor Qi. Here, when an overvoltage isapplied to the terminal “i,” the transistor Qi outputs an overvoltagedetection signal Si through its collector.

Next, the operation of the circuit described above will be described.Now, suppose that to the terminal “i” is applied a voltage Vi higherthan the sum of the voltages Vq1, VT4, and VT5 that respectively turnthe transistors Qi1, Tr4, and Tr5 on and the Zener voltages VD1, VD2,and VD3 that respectively turn on the Zener diodes ZD1 to ZD3. Then, acurrent I flows from the collector of the transistor Qi1 through theZener diodes ZD1 to ZD3 and then through the transistors Tr4 and Tr5 toground. Simultaneously, from the collector of the transistor Qi2, whichtogether with the transistor Qi1 forms a current mirror circuit, acurrent Ii proportional to the current I flows through the resistor Rito ground. This causes a voltage drop to occur across the resistor Riand thereby turns the transistor Qi on, making it output an overvoltagedetection signal Si.

As described above, in the first embodiment of the invention, the firstload portion 1 of the current mirror circuit connected to one of theterminals of the semiconductor integrated circuit device is providedwith an overvoltage protection circuit composed of Zener diodes, and isshared as the first load portions of the current mirror circuitsconnected to the other terminals. On the other hand, the second loadportions 2 a to 2 m of all the current mirror circuits are each providedwith an overvoltage detection circuit composed of a resistor and atransistor. This eliminates the need to provide many Zener diodes foreach terminal. Even then, when an overvoltage is applied to a terminal,it is possible to securely protect the internal circuit connected to theterminal, and it is also possible to detect an overvoltage terminal byterminal. Moreover, the protection voltage does not vary from terminalto terminal, and therefore it is possible to achieve highly accurateprotection against and detection of an overvoltage.

Next, with reference to FIGS. 2 and 3, a second embodiment of theinvention will be described. In the configuration shown in FIG. 2, fusesare used in place of the circuits that output overvoltage detectionsignals.

FIG. 2 shows one example of the circuit configuration of the secondembodiment. Here, the resistor Ri and the transistor Qi of the secondload portion 2 i are replaced with a fuse Fi. In the semiconductorintegrated circuit device 21 configured in this way, for example, whenan overvoltage is applied to the terminal “i,” the transistors Qi1 andQi2 forming a current mirror circuit, the Zener diodes ZD1 to ZD3, andthe transistors Tr4 and Tr5 operate just as described earlier. Here, byarranging the fuse Fi, which blows when the current Ii flows through thecollector of the transistor Qi2, in the second load portion 2 i providedon the downstream side thereof, it is possible to use the fuse Fi as astorage means for storing whether an overvoltage has ever been detectedor not. This makes it possible to inspect the individual terminals laterto identify those to which an overvoltage has ever been applied.

FIG. 3 shows another example of the circuit configuration of the secondembodiment. The configuration and operation here are the same as in thefirst embodiment except that the overvoltage detection signal Si is fedto a memory cell 3 provided as a storage means within the semiconductorintegrated circuit device 22. Here, the overvoltage detection signal Sioutput from the transistor Qi is stored in the memory cell as a historyof overvoltages applied to the terminal “i.” Thus, by inspecting theindividual terminals later, it is possible to identify those to which anovervoltage has ever been applied. If the memory cell is so configuredas to store the number of times that overvoltages have been detected ateach terminal and is furnished with a calendar function, it is possibleto store the dates and times that overvoltages were detected as ahistory that can be read out by the use of an unillustrated readingcircuit.

In the first and second embodiments, the first load portion 1 iscomposed of PNP- or NPN-type bipolar transistors and three Zener diodes.However, the first load portion 1 may be configured in any other mannerthan specifically described above; for example, it may include,depending on the type of the semiconductor integrated circuit device, aP- or N-channel MOS transistor or any number of Zener diodes or diodes.

Next, with reference to FIGS. 4A to 4C and 5A to 5E, a third embodimentof the invention will be described. FIGS. 4A to 4C show a protectiondiode used as a protection against an overvoltage applied to the supplyvoltage of the semiconductor integrated circuit device. Morespecifically, FIG. 4A is an equivalent circuit, FIG. 4B is a sectionalview of the semiconductor integrated circuit device, and FIG. 4C is atop view thereof. FIGS. 5A to 5E show a structure in which twotransistors forming a current mirror circuit are formed in the structureof the diode shown in FIGS. 4A to 4C formed in the semiconductorintegrated circuit device. FIG. 5A is an equivalent circuit, FIG. 5B isa sectional view of the semiconductor integrated circuit device, FIG. 5Cis a top view thereof, FIG. 5D shows practical example 1 of a circuit towhich the third embodiment is applied, and FIG. 5E shows practicalexample 2 of such a circuit.

In a semiconductor integrated circuit device, a surge voltage resultingfrom static electricity or the like occasionally appears at asupply-power or ground terminal or at any other terminal. For thisreason, in a conventional circuit, in general, the cathode electrode ofa protection diode D1 as shown in FIG. 4A is connected to a supplyvoltage terminal Vcc or any other terminal that is likely to be exposedto a surge voltage, and the anode electrode 4 of the protection diode D1is grounded through a resistor or the like. This permits a surge voltagehigher than the breakdown voltage of the diode to be absorbed by thediode. An example of the structure of such a protection diode formed ina semiconductor integrated circuit device will be described withreference to FIGS. 4B and 4C. In this example, on a P-type semiconductorsubstrate, an N⁺-type buried layer is laid, and then an N⁻-type layer islaid. Further on top of this N⁻-type layer, the P-type anode electrode 4of the protection diode D1 is formed, and then the N-type cathodeelectrode 5 thereof, which is connected to the supply power terminalVcc, is formed so as to surround the anode electrode 4.

In the third embodiment, in the structure described above, to detect anovervoltage, as shown in FIG. 5A, PNP-type transistors Q1 and Q2 thatform a current mirror circuit are formed within the region surrounded bythe cathode electrode 5 of the diode D1.

FIGS. 5B and 5C show an example of the structure of the semiconductorintegrated circuit device. In this example, within the region surroundedby the cathode electrode 5, which is connected to the supply voltageterminal Vcc, of the protection diode D1 described above with referenceto FIGS. 4A to 4C, the collector electrodes Ca and Cb of two independentP-type transistors Q1 and Q2 are formed so as to again surround theanode electrode 4 of the protection diode D1, and, parallel to thesecollector electrodes Ca and Cb, an N-type common base electrode Bs ofthe transistors Q1 and Q2 is formed. In this way, the transistors Q1 andQ2 having their bases connected together and having their emittersconnected together are formed.

FIG. 5A is an equivalent circuit of FIGS. 5B and 5C. In actual use, asshown in FIG. 1, the protection voltage may be raised by connectingZener diodes or transistors on the downstream side of the collectorelectrode Ca, and an overvoltage detection signal may be output byconnecting the collector electrode Cb to the second load portioncomposed of a resistor and an output circuit composed of a transistor.That is, the circuit configuration may be modified to suit the intendedpurpose. In that case, when an overvoltage is applied to this circuitvia the terminal “a” or the supply voltage terminal Vcc, it causes acurrent to flow through the protection diode D1 and through theunillustrated circuit elements connected on the downstream side of thecollector of the transistor Q1 to ground. Simultaneously, a currentproportional to that current flows through the collector of thetransistor Q2 and through unillustrated circuit elements to ground.Thus, this current can be detected to output an overvoltage detectionsignal.

As in practical example 1 shown in FIG. 5D, the circuit configuration ofthis embodiment may be applied to a plurality of terminals that arelikely to be exposed to an overvoltage. Here, ZD₁-ZD(n) represent that nZener diodes are connected together according to the magnitude of theexpected overvoltage. Moreover, here, the diode D2 that functions as aprotection diode is shown as a parasitic diode that appears in thesemiconductor integrated circuit device, and is therefore indicated withbroken lines. Alternatively, as practical example 2 shown in FIG. 5E,i.e., as in the first embodiment, the Zener diodes ZD₁-ZD(n) forabsorbing an overvoltage may be inserted in the first load portion la ofthe current mirror circuit connected to one of terminals “a” to “m” sothat this first load portion 1 a is shared as those of the currentmirror circuits connected to the other terminals.

In the third embodiment, when a protection diode is formed in asemiconductor integrated circuit device, transistors forming a currentmirror circuit are formed simultaneously within the region in which theprotection diode is formed. This makes it possible to form a protectioncircuit for protecting an internal circuit from an overvoltage appliedthereto via a terminal of the semiconductor integrated circuit devicetogether with a current mirror circuit in a small area, contributing tominiaturization of the semiconductor integrated circuit device.

As described above, with a semiconductor integrated circuit deviceincorporating an overvoltage detection circuit according to the presentinvention, even if a plurality of terminals are likely to be exposed toan overvoltage, by sharing a protection circuit against an overvoltage,it is possible to protect the terminals individually from an overvoltageand output an overvoltage detection signal with a reduced number ofcircuit elements. Moreover, it is possible to miniaturize thesemiconductor integrated circuit device as a whole, reduce costs, andreduce variations in the detection voltage resulting from variations inthe characteristics of the circuit elements used.

1-8. (canceled)
 9. A method of fabricating a semiconductor integratedcircuit device including a plurality of circuits, each composed of aprotection diode and a current mirror circuit consisting of first andsecond transistors having bases thereof connected together, havingemitters thereof connected together, and having the emitters thereofconnected to an anode of the protection diode, and a plurality ofterminals connected one to one to the circuits, comprising the steps of:forming the anode of the protection diode on the semiconductorintegrated circuit device; forming a cathode of the protection diode soas to surround the anode thereof; forming a collector of the firsttransistor so as to surround about a half of the anode within a regionsurrounded by the cathode; forming a collector of the second transistorso as to surround about another half of the anode; and forming the basesof the first and second transistors as a common base so as to besubstantially parallel to an edge of the collectors of the first andsecond transistors.
 10. A method of fabricating a semiconductorintegrated circuit device as claimed in claim 9, wherein thesemiconductor integrated circuit device is fabricated by laying anN⁻-type semiconductor layer on a P-type semiconductor substrate with anN⁺-type buried layer interposed in between, and on top forming the anodeof the protection diode with an N-type semiconductor, forming thecathode of the protection diode with an N-type semiconductor so as toreach the embedded layer; forming the collectors of the first and secondtransistors individually with a P-type semiconductor; and forming thecommon base of the first and second transistors with an N-typesemiconductor.